The present invention generally relates to surface-acoustic-wave (SAW) devices and more particularly to a SAW device having an improved pass-band characteristic. Further, the present invention relates to a SAW device that is flexible in design for setting input and output impedances of the SAW device as desired.
SAW devices are used extensively for a filter or a resonator in compact radio telecommunication apparatuses operational in a VHF or UHF band, a typical example being a portable telephone apparatus operational in a MHz band or GHz band.
In such high frequency radio telecommunication apparatuses, it is required that the SAW filters or SAW resonators used therein have a wide pass-band and simultaneously a very sharp off-band attenuation. Further, the SAW filters and resonators should be able to achieve an impedance matching with a cooperating circuit, which may be an integrated circuit forming the electronic apparatus in which the SAW device is used.
FIGS. 1A and 1B show the construction of a typical conventional SAW filter.
Referring to FIG. 1A, the SAW filter is a device of the so-called double-mode type and includes a pair of reflectors 10A and 10B on a piezoelectric substrate 1 as usual in a SAW filter, wherein the piezoelectric substrate may be a Y-X cut single-crystal plate of LiTaO.sub.3 or LiNbO.sub.3. Further, electrodes 11A, 11B and 11C are provided consecutively between the foregoing reflectors 10A and 10B from the reflector 10A to the reflector 10B.
In the illustrated example of FIG. 1A, the substrate 1 is formed of a single-crystal plate of 36.degree. Y-X LiTaO.sub.3, and the reflectors 10A and 10B, aligned in an X-direction of the substrate 1, define a propagation path of a surface acoustic wave excited on the piezoelectric substrate 1. Each of the electrodes 11A, 11B and 11C includes a primary-side interdigital electrode such as an electrode (11A).sub.1, (11B).sub.1 or (11C).sub.1 and a secondary-side interdigital electrode such as an electrode (11A).sub.2. (11B).sub.2 or (11C).sub.2, wherein the primary-side electrode and the secondary-side electrode are disposed such that the electrode fingers of the primary-side electrode and the electrode fingers of the corresponding secondary-side electrode extend in respective, mutually opposing directions, as usual in an interdigital electrode. Thereby, the electrode fingers of the primary-side electrode and the electrode fingers of the secondary-side electrode are repeated alternately in the X-direction on the substrate 1 and intersect the path of the surface acoustic wave traveling in the X-direction on the substrate 1. The pitch of the electrode fingers is determined by a central frequency of the SAW filter to be formed as well as by the sound velocity of the surface acoustic wave traveling on the substrate 1 in the X-direction. When viewed in the X-direction, the electrode fingers of the primary-side electrode and the electrode fingers of the secondary-side electrode overlap with each other over an overlap width W.
In the construction of FIG. 1A, the primary-side electrode (11A).sub.1 of the electrode 11A is connected to an input terminal commonly with the primary-side electrode (11C).sub.1 of the electrode 11C. On the other hand, the secondary-side electrodes (11A).sub.2 and (11C).sub.2 are both grounded. Thereby, the SAW filter of FIG. 1A forms a device of the so-called dual-input single-output type.
The double-mode SAW filter of such a construction uses a first-order mode of surface acoustic wave formed between the foregoing reflectors 10A and 10B with a frequency f.sub.1 and a third-order mode of surface acoustic wave formed also between the reflectors 10A and 10B with a frequency f.sub.3, wherein the SAW filter forms a pass-band characteristic as indicated in FIG. 2. FIG. 2 shows the attenuation of the SAW filter as a function of the frequency. In FIG. 2, it should be noted that a pass-band is formed between the foregoing frequency f.sub.1 of the first-order mode and the frequency f.sub.3 of the third-order mode. FIG. 1B shows the energy distribution of the surface acoustic wave excited in the structure of FIG. 1A.
Conventionally, it has been practiced to form the interdigital electrodes 11A-11C to be generally symmetric about the center of the X-axis in view of the corresponding symmetricity of the first-order mode and the third-order mode of the excited surface acoustic waves (see FIG. 1B), so that the first order-mode surface acoustic wave and the third-order-mode surface acoustic wave are excited efficiently. Thus, it has been practiced conventionally to set a number N.sub.1 indicating the number of the electrode finger pairs formed by the primary-side electrode fingers and the secondary-side electrode fingers in the interdigital electrode 11A, to be equal to a number N.sub.3 indicating the number of the electrode finger pairs formed by the primary-side electrode fingers and the secondary-side electrode fingers in the interdigital electrode 11C (N.sub.1 =N.sub.3).
However, FIG. 2 clearly indicates that various spurious peaks exist in the SAW device outside the pass-band defined by the frequencies f.sub.1 and f.sub.3. As a result of the existence of such spurious peaks, it should be noted that the sharpness of attenuation of surface acoustic wave outside the pass-band is reduced unwontedly, particularly in the frequency range between 1550 MHz and 1600 MHz. It should be noted that the attenuation of a SAW filter or resonator should be flat and minimum inside the passband and increase sharply outside the pass-band. In order to maximize the selectivity of the filter, it is desired to maximize the attenuation outside the passband.
In the conventional SAW filter of FIG. 1A, all of the interdigital electrodes 11A, 11B and 11C have the same overlap width W of the electrode fingers. Thus, the input and the output impedances of the SAW filter are determined by the number of pairs of the electrode fingers in the electrodes 11A-11C. Generally, it should be noted that the input and output impedances of a SAW filter are inversely proportional to the number of the electrode finger pairs N.sub.1 and N.sub.3 and the overlapping W for the electrodes 11A-11C. As the number N.sub.1 and the number N.sub.3 of the electrode finger pairs are set equal to each other and the overlap width W is constant in conventional SAW devices, it has been difficult to set the input impedance and the output impedance independently and as desired. Thus, conventional SAW devices have failed to meet the demand for the capability of flexibly setting the input and output impedances, while such a demand of flexible setting of the input and output impedances is particularly acute in recent compact radio apparatuses for GHz applications such as a portable or mobile telephone apparatus.